Combined electrically heatable converter

ABSTRACT

An improved converter having electrically heatable strips connected in an electrical circuit, and intertwined with non-electricvally heated thin metal strips, having an electrical connection, or anchor, from the electrically heatable thin metal strips to the non-electrically heated thin metal strips, whereby the potential difference between the electrically heatable strips non-electrically heatable thin metal strips is reduced.

This invention relates to a combined electrically heatable and anon-electrically heatable converter, and more particularly to animproved combined electrically heatable and light-off converter in whichthe electrically heatable portion is electrically connected to the"light-off" portion. The purpose of the connection is to reduce thepotential difference between the thin metal leaves or layers making upthe electrically heatable portion and the thin metal leaves or layersmaking up the "light-off" portion. The thin metal layers of bothportions are coated, usually with a refractory metal oxide coating,e.g., alumina-containing, and the dielectric strength of the coatings isoftentimes insufficient to withstand the voltages impressed on theelectrically heatable portion. The present invention improves theconverter by reducing the tendency to arcing or shorting through fromthe electrically heated thin metal layers to the nonelectrically heatedthin metal layers of, for example, the "light-off" portion. Morespecifically, the electrically heated thin metal layers are intertwinedwith the non-heated thin metal layers.

BACKGROUND OF THE INVENTION AND PRIOR ART

This invention is, as indicated, an improvement on a combinedelectrically heatable converter including an electrically heatableportion and a "light-off" portion, each portion being made up ofcorrugated thin metal strips or layers in alternating relation with flatthin metal strips or layers. Electrical power is supplied to theelectrically heatable portion only. Such devices are described incommonly owned U.S. patent applications Ser. Nos. 08/013,516 filed 3Feb. 1993 by Whittenberger and Woodruff, now abandoned and 08/066,887filed 25 May 1993 by Whittenberger, now abandoned. These applicationsare incorporated herein by reference thereto.

The invention will be described in connection with embodimentsespecially adapted for use in exhaust lines from various types ofengines, e.g., internal combustion engines of the spark ignited orcompression ignited types, stationary or mobile, or gas turbine engines.It will be understood, however, that the converters of the presentinvention may be used to effect various chemical reactions, particularlythose occurring in fluid streams, especially gas streams, whichreactions are catalyzed or uncatalyzed.

Turning now to converters especially useful in exhaust lines extendingfrom internal combustion engines, e.g., those used in automotivevehicles, the purpose of such catalytic converters is to convertpollutant materials present in the exhaust stream, e.g., carbonmonoxide, unburned hydrocarbons, ozone, nitrogen oxides, etc., to carbondioxide, nitrogen oxygen and water prior to discharge to the atmosphere.Conventional automotive catalytic converters utilize an oval or circularcross-section ceramic honeycomb monolith having square or triangularstraight-through openings or cells with catalyst deposited on the wallsof the cells; catalyst coated refractory metal oxide beads or pellets,e.g., alumina beads; or a corrugated thin metal foil multicelledhoneycomb monolith having a refractory metal oxide coating and catalystcarried on the coating and supported on the surfaces of the cells. Thecatalyst is normally a noble metal, e.g., platinum, palladium, rhodium,ruthenium, or a mixture of two or more of such metals. Zeolite coatingsmay also be used for the adsorption and desorption of pollutants to aidin their removal. The catalyst catalyzes a chemical reaction, mainlyoxidation, whereby the pollutant is converted to a harmless by-productwhich passes through the exhaust system to the atmosphere.

However, conversion to such harmless by-products is not efficientinitially when the exhaust gases are relatively cold, e.g., at coldengine start. To be effective at a high conversion rate, the catalystand the surface of the converter must be at or above a minimumtemperature, e.g., 390 degrees F. for carbon monoxide, 570 degrees F.for volatile organic compounds, and about 900 degrees F. for methane ornatural gas. Otherwise, conversion to harmless by-products is poor andcold start pollution of the atmosphere is high. It has been estimatedthat as much as 80% of the atmospheric pollution caused by vehicles eventhough equipped with conventional non-electrically heated catalyticconverters, occurs in the first two minutes of operation of the enginefrom cold start. Once the exhaust system has reached its normaloperating temperature, a non-electrically heated catalytic converter isoptimally effective. Hence, it is necessary for the relatively coldexhaust gases to make contact with hot catalyst so as to effectsatisfactory conversion. Compression ignited engines, spark ignitedengines, reactors in gas turbines, small bore engines such as used inlawn mowers, trimmers, boat engines, and the like have this need.

To achieve initial heating of the catalyst at engine start-up, there isconveniently provided an electrically heatable catalytic converter unit,preferably one formed of a thin metal honeycomb monolith. The monolithmay be formed of spaced flat thin metal strips, straight-throughcorrugated thin metal strips, pattern corrugated thin metal strips,e.g., herringbone or chevron corrugated thin metal strips, or variablepitch corrugated thin metal strips (such as disclosed in U.S. Pat. No.4,810,588 dated 7 Mar. 1989 to Bullock et al) or a combination thereof,which monolith is connected to a 12 volt to 108 volt or higher AC or DCvoltage source, single or multi-phase, preferably at the time of enginestart-up and afterwards to elevate the catalyst to and maintain thecatalyst at a temperature of at least 650 degrees F. plus or minus 30degrees F. Alternatively, power may also be supplied for a few secondsprior to engine start-up.

Catalytic converters containing a corrugated thin metal (stainlesssteel) monolith have been known since at least the early 1970's. SeeKitzner U.S. Pat. Nos. 3,768,982 and 3,770,389 each dated 30 Oct. 1973.More recently, corrugated thin metal monoliths have been disclosed inU.S. Pat. No. 4,711,009 dated 8 Dec. 1987 to Cornelison et al; U.S. Pat.Nos. 4,152,302 dated 1 May 1979, 4,273,681 dated 16 Jun. 1981, 4,282,186dated 4 Aug. 1981, 4,381,590 dated 3 May 1983, 4,400,860 dated 30 Aug.1983, 4,519,120, dated 28 May 1985, 4,521,947 dated 11 Jun. 1985,4,647,435 dated 3 Mar. 1987, 4,665,051, dated 12 May 1987 all toNonnenmann alone or with another; U.S. Pat. No. 5,070,694 dated 10 Dec.1991 to Whittenberger; International PCT Publication Numbers WO 89/10470(EP 412,086) and WO 89/10471 (EP 412,103) each filed 2 Nov. 1989claiming priority date of 25 Apr. 1988. The above InternationalPublication Numbers disclose methods and apparatus for increasing theinternal resistance of the device by placing spaced discs in series, orelectrically insulating intermediate layers. Another International PCTPublication is WO 90/12951 published 9 Apr. 1990 and claiming a prioritydate of 21 Apr. 1989, seeks to improve axial strength by form lockinglayers of insulated plates. Another reference which seeks to improveaxial strength is U.S. Pat. No. 5,055,275 dated 8 Oct. 1991 toKannainian et al. Reference may also be had to PCT Publication Number WO92/13636 filed 29 Jan. 1992 claiming a priority date of 31 Jan. 1991.This application relates to a honeycomb body along an axis of whichfluid can flow through a plurality of channels. The honeycomb has atleast two discs in spaced relation to each other. According to thespecification, there is at least one bar type support near the axis bywhich the discs are connected together and mutually supported. Theinvention is said to make possible design of the first disc for fastheating up through exhaust gas passing through or applied electricalcurrent. The honeycomb body serves as a bearer for catalyst in theexhaust system of an internal combustion engine. Another reference isGerman Patent Application Number 4,102,890 A1 filed 31 Jan. 1991 andpublished 6 Aug. 1992. This application discloses a spirally woundcorrugated and flat strip combination wherein the flat strip containsslots and perforations and is electrically heatable. The flat stripsinclude a bridge between leading and trailing portions. Groups of suchstrips are separated by insulation means. The core is provided with apair of circular retainer segments which are separated by insulationmeans. Another reference is U.S. Pat. No. 5,102,743 dated 7 Apr. 1992 toMaus. This patent discloses a honeycomb catalyst carrier body of round,oval or elliptical cross-section including a jacket tube and a stack ofat least partially structured sheet metal alyers intertwined indifferent directions in the jacket tube. The stack has a given lengthand a given width. At least one of the sheet metal layers has a greaterthickness over at least part of one of the dimensions than others of thelayers. Such at least one layer is formed of thicker metal or of aplurality of identically structured metal sheets in contiguous relation.

Most recently, combined electrically heatable and "light-off" convertershave been provided in which electrically heatable flat thin metal stripsare alternated with non-electrically heatable corrugated thin metalstrips and spirally would in a jacket tube to provide what is called anintegral structure, that is, where the electrically heatable portion istied to the "light-off" portion by interleafing the corrugated thinmetal sheets with the flat, electrically heatable thin metal sheets, andintertwining the assembly, as by spirally winding, into a suitablehousing or jacket tube. Such structures are disclosed in the aforesaidSer. Nos. 08/013,516 and 08/066,887, both abandoned.

A common problem with prior devices has been their inability to survivesevere automotive industry durability tests which are known as the HotShake Test and the Hot Cycling Test.

The Hot Shake Test involves oscillating (100 to 200 Hertz and 28 to 60 Ginertial loading) the device in a vertical attitude at a hightemperature (between 800 and 950 degrees C.; 1472 to 1742 degrees F.,respectively) with exhaust gas from a running internal combustion enginesimultaneously passing through the device. If the catalytic devicetelescopes or displays separation or folding over of the leading orupstream edges of the foil leaves up to a predetermined time, e.g., 5 to200 hours, the device is said to fail the test. Five hours is equivalentto 1.8 million cycles at 100 Hertz.

The Hot Cycling Test is one with exhaust gas flowing at 800 to 950degrees C.; 1472 to 1742 degrees F.) and cycled to 120 to 150 degrees C.once every 15 to 20 minutes, for 300 hours. Telescoping or separation ofthe leading edges of the thin metal foil strips is considered a failure.

The Hot Shake Test and the Hot Cycling Test are hereinafter called "HotTests" and have proved very difficult to survive. Other durability testsare known for EHC's including an electrical cycling test. The EHC isheated to 500-700 degrees C. at rated power and then fan cooled, cyclingevery 1-3 minutes. This is done for 50,000 cycles. There should be nochanges in electrical resistance.

The structures of the present invention will survive these Hot Tests.

In the following description, reference will be made to "ferritic"stainless steel. A suitable ferritic stainless steel for useparticularly in the engine exhaust applications hereof is described inU.S. Pat. No. 4,414,023 dated 8 Nov. 1983 to Aggen. A specific ferriticstainless steel alloy useful herein contains 20% chromium, 5% aluminum,and from 0.002% to 0.05% of at least one rare earth metal selected fromcerium, lanthanum, neodymium, yttrium, and praseodymium, or a mixture oftwo or more of such rare earth metals, balance iron and trace steelmaking impurities. A ferritic stainless steel is commercially availablefrom Allegheny Ludlum Steel Co. under the trademark "Alfa IV." Anothermetal alloy especially useful herein is identified as Haynes 214 alloywhich is commercially available. This alloy and other nickeliferousalloys are described in U.S. Pat. No. 4,671,931 date 9 Jun. 1987 toHerchenroeder et al. A specific example contains 75% nickel, 16%chromium, 4.5% aluminum, 3% iron, optionally trace amounts of one ormore rare earth metals except yttrium, 0.05% carbon and steel makingimpurities. Haynes 230 alloy, also useful herein, has a compositioncontaining 22% chromium, 14% tungsten, 2% molybdenum, 0.10% carbon, anda trace amount of lanthanum, balance nickel. The ferritic stainlesssteels and the Haynes alloys 214 and 230 are examples of hightemperature resistive, oxidation resistant (or corrosion resistant)metal alloys that are suitable for use in making thin metal strips foruse in the converter bodies hereof, and particularly for making heaterstrips for the EHC (electrically heatable converter) portions and"light-off" portions hereof. Suitable metals must be able to withstand"high" temperatures of 900 degrees C. to 1200 degrees C. (1652 degreesF. to 2012 degrees F.) over prolonged periods.

Other high temperature resistive, oxidation resistant metals are knownand may be used herein. For most applications, and particularlyautomotive applications, these alloys are used a "thin" metal strips,that is, having a thickness of from about 0.001" to about 0.005", andpreferably from 0.0015" to about 0.003".

BRIEF STATEMENT OF THE INVENTION

Briefly stated, the present invention is an improvement in a converterhaving an electrically heatable portion and a non-electrically heatedportion. The electrically heatable portion has at least one thin metalheater strip connected in an electrical circuit and is electricallyheatable through terminal connections to a voltage source. Thenon-electrically heated portion has at least one corrugated thin metalstrip alternating with flat thin metal strips which are not electricallyheatable and which corrugated thin metal strip is intertwined with saidheater strip and carried in a housing. Normally, the corrugated stripsare secured to the housing and thus at ground potential. The improvementis, in lieu of a ground connection, an electrical connection from theheater strip circuit at a point intermediate the terminal connections tosaid voltage source, to the ungrounded corrugated thin metal strip,whereby the electrical potential of the corrugated thin metal strip isheld to a potential between the potentials at the terminal connections.The thin metal strips are desirably coated as will be specificallypointed out below.

For example, if a coated thin metal heater strip is assumed as connectedto a 50 volt DC voltage source, one end can be viewed as having apotential of zero, while the opposite end of the strip is at +50 volts.A contiguous coated corrugated thin metal strip not being connected inany electrical circuit and not being grounded by attachment to thehousing, (i.e., insulated from said housing as for example, disclosed incommonly owned copending application Ser. No. 08/164,038 filed 9 Dec.1993), would see a potential difference adjacent the opposite end of theheater strip of 50 volts. This potential difference is normallysufficient to arc through the coating and ultimately damage theconverter body. However, if, as set forth in this invention, anelectrical connection is provided at an intermediate point in thecircuit, for example, the midpoint of the heater strip to any point onthe corrugated strip, imposing a potential of 25 volts on the corrugatedstrip, then the potential difference at the opposite end is only 25volts (50-25) which potential difference is insufficient to breakthrough the coating.

The invention also contemplates a method of reducing the potentialdifference between a point along a heater strip connected in anelectrical circuit for purposes of heating said heater strip, and acontiguous coated corrugated thin metal strip which is not groundedthrough the housing which comprises "anchoring" the heater strip to thecorrugated strip by connecting one to the other with an electricalconductor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood by having reference to theannexed drawings wherein:

FIG. 1 is an isometric perspective view of core body of the integraltype having flat electrically heated thin metal strips forming anelectrically heatable portion intertwined with corrugated thin metalstrips which are not electrically heated and extend into a "light-off"portion, and showing a segmented retaining shell for the EHC portion anda fully tubular shell or housing in the "light-off" portion.

FIG. 2 is a fragmentary end view on an enlarged scale of an integraltype EHC and "light-off" converter such as shown in FIG. 1, showing thecells formed by the alternating flat and corrugated thin metal strips incontiguous relation.

FIG. 3 shows in diagrammatic and schematic form a single circuit designfor an anchored circuit.

FIG. 4 shows in diagrammatic and schematic form a two-circuit design foran anchored circuit.

FIG. 5 is an electrical schematic for an anchored core design.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is here shown a perspective view of aconverter body in accordance with this invention and showing the dualnature of the electrically heatable devices hereof. The front portion 24is defined by an S-wound core generally indicated at 21 retained in apair of arcuate members 22 and 20. The arcuate members 22 and 20 areelectrically isolated, one of which is adapted to be attached to onepole of a voltage source, not shown, e.g., a 12 volt storage battery,and the other of which is adapted to be attached to the other pole ofsaid storage battery. In the front portion 24 are shown the upstreamends of the electrically heatable flat thin metal strips 14a, theupstream ends of the flat thin metal strips 14 which are notelectrically heated, and the upstream ends of the corrugated thin metalstrips 12. In regard only to the flat strips, alternate flat strips 14extend the entire axial length of the converter body of FIG. 1, whereasthe heatable flat thin metal strips 14a in alternating relation with theflat thin metal strips 14 extend only a relatively short axial distance,e.g., 0.25" to 2" into the converter body. The remaining portions of theflat strips extend the remaining axial distance of the converter body ofFIG. 1, and in combination with the corrugated thin metal strips 12define the "light-off" portion of the converter. The flat thin metalstrips 14a are spaced from and out of electrical contact with theremaining portions, the axial spacing distance being from about 0.05" toabout 0.25", e.g., 0.075". It will be seen, therefore, that the EHCportion and the "light-off" portion of the converter bodies hereof areindeed integral because, for example, the several corrugated thin metalstrips 12 may extend the entire axial length of the converter body,while the heater bands 14a in the EHC portion extend only a relativelyshort axial distance into the converter body and are then separated fromthe balance of the flat strip. The core body contained in an innerhousing 24, 26 shown in FIG. 1 is adapted to be contained in an outerhousing, not shown, but clearly shown and described in the aforesaidU.S. Ser. No. 08/066,887 incorporated herein by reference.

FIG. 2 shows a fragment of an end view on an enlarged scale of theelectrically heatable core body hereof showing the alternatingrelationship of the flat and corrugated thin metal strips. Corrugatedthin metal strips 30 are shown in alternating relation with flat thinmetal strips 32, and together they define cells 34 through which exhaustgas flows. The corrugated thin metal strips do not carry current. In theanchored circuit hereof, the electrical potential of any point on thecorrugated thin metal strips or leaves will be intermediate, e.g., themidpoint, the two terminal potentials. The flat heater strips 32 carrycurrent. The electrical potential at a given point on the strips 32 maybe the potential on the potential gradient between the two terminalpotentials. The potential gradient may extend, for example, from 0 voltsat one terminal to 50 volts at the other terminal. The anchor connectionmay be, for example, at the midpoint, e.g., 25 volts. As indicatedabove, the thin metal foil strips 30 and 32 are coated with a coating.Thus on flat strip 32, the coating 36 is on one side and the coating 37is on the opposite side. With the corrugated thin metal strips 30, thereis provided a coating 38 on one side and a coating 40 on the other side.In the preferred case, the coatings 36, 37, 38 and 40 are put on thestrips prior to assembly. See U.S. Pat. No. 4,711,009 supra toCornelison et al. These coatings are very thin, e.g., 0.00005" to 0.002"thick. The coatings 36, 37, 38, and 40 are conveniently a refractorymetal oxide, e.g., alumina, alumina/ceria, titania, titania/alumina,titania/ceria, silica, zirconia, zirconia/alumina, etc. While thesecoatings may be used on both the corrugated and flat thin metal strips,the electrically heatable flat thin metal strips are desirably coatedwith a "dielectric" coating, or a chrome oxide coating such as Bureau ofStandards Coating A-418.

A typical procedure for applying a "dielectric" coating involves firstcoating the thin metal strip, whether flat or corrugated, and preferablyformed of an aluminum-containing high temperature resistive, oxidationresistant alloy, with a single coat of gamma-alumina in an aqueousslurry at a loading of about 5 mg per square inch of surface. This stepis followed by drying and then calcining at 800 to 1200 degrees C. (1472to 2192 degrees F.) for at least about 10 seconds, to form a tightlybonded insulative coating, and then coating again with from 1 to 6 ormore coatings of gamma-alumina/ceria mixture to a loading of about 30 to40 mgs. per square inch, drying after each coat, and then calcining atabout 500 degrees C. (930 degrees F.). The usual coating with arefractory metal oxide produced at a lower temperature of 500 to 1200degrees F. also has insulating properties, but not to the same extent asthe "dielectric" coating. The "dielectric" coating is believed to be aspinel and thus chemically different from the normal refractory metaloxide calcined at a lower temperature. Both types of coatings may beused in a given embodiment of converter bodies of the present invention.The "dielectric" coated more elements are usually only the heater strips30 and 32 in FIG. 2, or 14a in FIG. 1. The "dielectric" coated stripshave a higher dielectric strength than the usual refractory metal oxidecoated strips.

Another useful insulative high temperature resistive coating is composedof a silica glass frit 46%, chrome oxide 19.6%, Black Label Clay 5% anddeionized water 31.4% applied as a slurry and calcined at approx. 950degrees C. for less than 5 minutes. This composition has a Bureau ofStandards number A-418 Coating. It may be applied by dip coating, or byspray application.

FIG. 3 shows a structure for the electrically heatable portion in whichthe corrugated strips 50 are connected at their midpoints by a weldment54, e.g., a spot weld, to the flat thin metal strips 52. Ends 56 of theflat thin metal strips 52 are attached to a terminal strip 58 which isin turn attached to the positive pole of a voltage source, not shown.The other ends 60 of the flat thin metal strips 52 are attached to aterminal strip 62 which is in turn attached to the negative pole of thevoltage source, not shown. The schematic of FIG. 3 is a single circuitdesign.

FIG. 4 shows a two-circuit design for the electrically heatable portionof the converters hereof. Here flat strips 70 and 72 are connected atends 75 and 77 to a terminal strip 74, which terminal strip is connectedin turn to the positive pole of a voltage source, not shown. Theopposite ends 78 and 80 are welded or brazed to an inner metallichousing represented by the connector strip 76 which is desirablyinsulated from any ground potential. Flat strips 82 and 84 are connectedat the ends 81 and 83 to the connector strip 76 and back to a terminalstrip 86 to which strips 82 and 84 are connected at their ends 87 and89. Terminal strip 86 is connected to the negative pole of a voltagesource, not shown. The corrugated strips are connected to the connectorstrip 76 at their proximal ends 92, which, so far as FIG. 4 isconcerned, is the midpoint of the circuit. The distal ends 94 of thecorrugated thin metal strips are not attached to any part of theelectrical circuit and current cannot flow through the corrugated thinmetal strips 90.

FIG. 5 is a schematic diagram of an electrical circuit in accordanceherewith. Here are shown a flat electrically heatable heater strip 100and a non-electrically heated corrugated strip 102. The wavy line 102represents the non-electrically heated corrugated thin metal strips andflat thin metal strips defining the "light-off" portion of theconverter. The heater strip 100, which may be corrugated, if desired, isconnected at one end to a terminal 104, and at the other end to aterminal 106. The potential at terminal 104 is indicated as zero voltsand the potential at terminal 106 is indicated as 50 volts. Theresistance of the heater strip 100 is indicated by the series connectedresistances 108, 110, 112, and 114. The corrugated foil 102 is connectedor anchored at 115 to the midpoint 116 of the heater strip 100 by ananchor connection 118. Thus, the leaf-to-leaf potential difference atthe end 120 is a maximum of 25 volts, and not 50 volts which it would bebut for the anchor connection 118. Also at the end 122, the potential is-25 volts with the anchor connection in place. The 50 volt potentialdifference would be sufficient to arc through the insulative coatings onthe respective strips 100 and 102 and ultimately damage the core of theconverter. The potential difference of 25 volts with the anchor 118 inplace is insufficient to break through the insulative coatings on therespective strips 100 and 102. The anchor 118 does not need to be placedat the midpoint of the respective strips 100 and 102 as shown in FIG. 5,but may be located at any point intermediate the ends 104 and 106 andconnect with an opposite point on the corrugated thin metal strip, e.g.,any point intermediate the ends 120 and 122. To attain the highestpossible resistance to arcing, the anchor 118 should be placed at themidpoint of the respective strips 100 and 102.

Reference may be had to co-pending commonly owned U.S. Pat. No.5,319,929 by Whittenberger for a suitable switching circuit fordelivering higher power to the electrically heatable portion of theconverter and using MOSFETs (metal oxide semiconductor field effecttransistors).

What is claimed is:
 1. In a converter for treating pollutant-containingexhaust gas having at least one electrically heatable thin metal stripconnected in an electrical circuit and being electrically heatablethrough terminal connections to a voltage source, intertwined with aplurality of non-electrically heated thin metal strips, the improvementwhich comprises an electrical connection from said at least one thinmetal strip in said electrical circuit at a point intermediate theterminal connections to said voltage source, to said non-electricallyheated thin metal strips, whereby the electrical potential of thenon-electrically heated thin metal strips is maintained at a potentialbetween the potentials at said terminal connections.
 2. A converter asdefined in claim 1 wherein the electrically heatable strips are flatthin metal strips and said non-electrically heated thin metal strips arecorrugated.
 3. A converter as defined in claim 2 wherein the flat thinmetal strips are in alternating relation with the corrugated thin metalstrips.
 4. A converter as defined in claim 3 wherein the potential atwhich the corrugated thin metal strips is held is one half the potentialbetween the terminal connections.
 5. A converter as defined in claim 3wherein the potential at which the corrugated thin metal strips is heldis one third the potential between the terminal connections.
 6. Aconverter as defined in claim 2 wherein the flat thin metal strips arecoated with an insulative coating.
 7. A converter as defined in claim 6wherein the insulative coating comprises a refractory metal oxide.
 8. Aconverter as defined in claim 6 wherein the insulative coating is a"dielectric" coating.
 9. A converter as defined in claim 6 wherein thethin metal strips also support a catalyst.
 10. A converter as defined inclaim 9 wherein the catalyst is a noble metal.
 11. A converter asdefined in claim 10 wherein the noble metal is selected from the groupconsisting of platinum, palladium, rhodium, ruthenium, indium, andmixtures of two or more of such metals.
 12. A converter as defined inclaim 2 wherein the corrugated thin metal strips are coated with aninsulative coating.
 13. A converter as defined in claim 12 wherein theinsulative coating comprises a refractory metal oxide.
 14. A converteras defined in claim 13 wherein the refractory metal oxide comprisesalumina.
 15. A converter as defined in claim 13 wherein the refractorymetal oxide is a mixture of alumina and ceria.
 16. A converter asdefined in claim 2 wherein the thin metal heater strips are coated witha "dielectric" coating and the corrugated thin metal strips are coatedwith a refractory metal oxide.
 17. A converter as defined in claim 1wherein the thin metal strips are a stainless steel.
 18. A converter asdefined in claim 1 wherein the thin metal strips are a ferriticstainless steel.